* {
    padding: 0;
    margin: 0;
}

a {
    list-style: none;
}

li {
    text-decoration: none;
}

.bottom,
.mask {
    width: 243px;
    height: 180px;
}
.bottom {
    position: relative;
    margin: 100px auto;
}
.mask {
    position: absolute;
    top: 0;
    left: 0;
    display: none;
    background:rgba(0, 0, 0, .2) url(../第七天/study/images/hot.jpg) no-repeat center center;
}

.bottom a:hover .mask{
    display: block;
}